Feedback circuits



March 20, 1945. c. MADSEN 2,372,101

' FEEDBACK CI RGUITS Filed Se t. 1, 1942 Anfermae Audio 519ml mpllfler WITNESSES: INVENTOR W Car/ JMQOSe/Z I fiwwi v Patented Mar. 2o,194s

FEEDBACK cracm'rs Carl J. Madsen, Baltimore, MIL, assign: to Westinghonse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsyl- Application September 1, 1942, Serial No. 458,892

4 Claims.

My invention relates to amplifiers for electric currents and, in particular, to arrangements for I eliminating distorting noises introduced by the physical properties of certain amplifier tubes in speech transmission circuits, particularly those of the radio type.

It is found, particularly in final amplifier tubes of radio transmitters of high power, that distortions, commonly referred to as tube-noise, are introduced into the transmitted waves by certain physical characteristics of the amplifier tubes.

current voltage from the impedance drop in said cathode lead of the amplifier and obtain a voltage" which is proportional to the instantaneous sum of the modulation plus the distorting noise due to the amplifier tube. By making use of the above-mentioned voltage derived from the secondary of the modulation transformer, I neutralize the modulation frequency component of This noise is usually of relatively small volume,

v and is noticeable only when pauses of two seconds or more duration occur in the speech, music or other signal being transmitted. Sometimes this distortion is produced by the magnetic effect of the current which heats the cathodes of the tube, but other causes of distortion due to the tubes likewise exist. It has been attempted in the prior art to reduce noise and distortion by feeding back a smallportion of the rectified modulated carrier current into one of the early audio amplifier stages, but this method proved inadequate in many instances, sometimes because of phase shift introduced by the modulation transformer and other elements in the coupling circuit between the modulator and the tube of which the output is being modulated.

I have discovered that a considerably simpler and more effective arrangement can be made for eliminating and neutralizing the distortion due to the amplifier tube noise above mentioned.

The principal object of my invention is, accordingly, to provide an arrangement for introducing a neutralizing voltage in a radio transmitter which shall eliminate the distortion introduc'ed into the transmitted waves by tubes in a 7 late amplifier stage.

My arrangement for thus neutralizing the effect of tube distortion is based on the discovery that the current flowing from the cathode to the negative terminal of the plate voltage source of an. output amplifier carrying the modulated carrier wave has a wave form which substantially consists of the modulated carrier wave plus the distortion components introduced by the amplifier tubesin question. I have also discovered that a voltage proportional from instant to instant to l the program modulated carrier wave can be derived from the terminals of the modulation transformer which impresses audio frequency modulation through a Heising connection to the plate circuit of the above-mentioned amplifier. By providing a proper bypass in the cathode lead of the amplifier, I substantially remove the carrier the last-mentioned voltage and obtain a voltage proportional to the distortion components introduced by the amplifier tube. This voltage I feed back to the audio amplifier system and thereby introduce a component voltage which neutralizes a large portion of the tube distortion noise in the output circuit of the amplifier.

The single figure of drawing illustrates one embodiment of my invention.

Referring in detail to the drawing, an amplifier tube 1, which is of any conventional type suitable for amplifying modulated carrier current for a high power transmitter and which may be of the triode type shown here, has an input circuit energized by carrier current connected between its control electrode 2 and its cathode 3. The cathode 3 is connected to ground through a capacitor 4, a resistor 5 and an audio choke inductance 6 connected in multiple with each other. The capacitor 4 is of such size as to oppose no impedance to the carrier current frequency but to impose substantial impedance to the signal modulation and to the distorting noises normally introduced in the tube output by the physical characteristics of the tube I. The anode l is connected through an inductance 8 to one secondary terminal of a transformer 9 on the primary of which a voltage proportional to the audio frequency signal modulation is impressed. The

other terminal of the secondary of the transformer 9 is connected through a capacitor ii to ground. The positive terminal I2 of a high voltage source having its negative terminal grounded is connected through a Heising choke coil Hi to the terminal of the inductance a which is remote from the anode 'l. The positive terminal I2 is likewise connected to ground through a bypass capacitor is in accordance with conventional practice. The two terminals of the inductance 8 are connected through a pair of capacitors l5, IS with' a tuned network comprising a variable ,capacitor I'l shunted by the primary l8 of an output transformer having a secondary winding l9 connected to supply energy to an output antenna'or other work circuit. The common terminal of the capacitors l6, H is grounded.

S'nunted across the remote terminals of the secondary of the transformer 9 and the blocking capacitor ll I connect a resistor H in series with a blocking capacitor 22 and provide a variable tap 23 so that a regulative voltage may be derive'd from the resistor 2! which is any submultiple of the output voltage of the modulation transformer 9. The tap 23 is connected through the primary zfl of an audio frequency transformer to the cathode 3 of the tube i. The secondary 25 cooperating with the primary winding 2a is connected preferably through a pair of resistors 26, 27 to the terminals of a capacitor 28, the terminals of which are similarly connected through a pair of resistors 29, 3! to impress the voltage derived from the secondary winding 25 on the input circuitof an audio amplifier supplying energy to the modulation transformer 9. The system 26, 21, 28, 29, 3| constitutes an attenuating and isolating network. I have found that the most prominent noise and distortion components are between 50 and 600 cycles and the capacitor 28 may, therefore, be of such value as to provide substantial impedance to the flow of currents of this frequency, but to offer comparatively slight impedance to frequencies of the order of 4000 ,or above in the modulating wave which might otherwise be fed back through the windings 24, 25 to the output of the modulation transformer.

As already stated, the voltage appearing between the cathode 3 and the ground (which latter is the negative terminal of the plate current voltage source of tube I) is substantially proportional instant by instant to the sum of the modulation voltage plus the distorting waves introduced by the physical characteristics of the tube I. By adjusting the movable tap 23 in the resistor 2!, I can obtain a voltage which is exactly equal to the modulation component in the voltage across the terminals of the resistor 5. If desired, I can make this adjustment by measuring, by methods well known in the art, the audio frequency component of the voltage appearing across the terminals of primary winding 24 while I am adjusting the position of the tap 23, and moving the latter until this voltage is substantially zero. When the system is so adjusted, the voltage supplied from the output terminals of the resistors 29, 3| to the audio frequency amplifier will be proportional to the distortion and noise introduced in the system by the tube I. By impressing the voltage supplied by the resistors 29,

{3| across a variable tap on a potentiometer located in the input circuit of an audio amplifier tube, and moving this variable tap until distortion voltages in the output circuit I9 are a minimum, I substantially eliminate and neutralize such distortion voltages.

I claim as my invention:

1. In combination with an amplifier tube having an output electrode, an input electrode 'and a cathode, an input circuit supplied with carrier frequency connected between said cathode and said input electrode, an output circuit, a network portional to the instantaneous difference between the voltage across said network and a fraction of said modulation frequency voltage, and means for impressing the last-mentioned voltage on the input of said audio frequency amplifier.

2. In combination with an amplifier tube'having an output electrode, an input electrode and a cathode, an output circuit, a network comprising a capacitor in multiple with an element offering substantial impedance to audio frequency currents in the cathode lead of said amplifier, means including an audio frequency amplifier for impressing audio frequency modulation voltages between said output electrode and the terminal of said network remote from said cathode and means for deriving a voltage proportional to the instantaneous difference between the voltage across said network and a fraction of said modulation frequency voltage, and means for impressing the last-mentioned voltage on the input of said audio frequency amplifier.

3. In combination with an amplifier tube hav ing an output electrode, an input electrode and a cathode, an input circuit supplied with carrier frequency connected between said cathode and said input electrode, an output circuit, a network comprising a capacitor in multiple with an element offering substantial impedance to audio frequency currents in the cathode lead of said amplifier, a source of plate voltage connected through a Heising coil between ground and said output electrode, the terminal of said network remote from said cathode being grounded, an audio frequency amplifier having an output circuit connected between the terminal of said Heising coil and ground, a potentiometer having a variable tap connected in series with a blocking condenser across the output of said audio frequency amplifier, a primary transformer winding connected between said cathode and said variable tap, a secondary windinzassociated with the lastmentioned primary winding connected to impress a voltage upon an input circultof said audio frequency amplifier.

4. In combination with an amplifier tube having an output electrode, an input electrode and a cathode, an input circuit supplied with carrier frequency connected between said cathode and said input electrode, an output circuit, a network comprising a capacitor in multiple with an element offering substantial impedance to audio frequency currents in the cathode lead of said amplifier, a source of plate voltage connected through a Heising coil between ground and said output electrode, the terminal of said network remote from said cathode being grounded, an

comprising a capacitor in multiple with an eleaudio frequency amplifier having an output circuit connected between the terminal of said Heising coil and ground, a potentiometer having a variable tap connected in series with a blocking condenser across the output of said audio frequency amplifier, a primary transformer winding connected between said cathode and said variable tap, a secondary winding associated with the lastmentioned primary winding connected through an attenuating network comprising series resistance in the line and a capacitor in shunt with the line to impress a voltage upon an input circuit of said audio frequency amplifier.

CARL J. MAD SEN. 

